Bottom Line:
Several new methods for the digital discrimination of neutrons and gamma-rays in a mixed radiation field are presented.The methods introduced discriminate neutrons and gamma rays successfully in the digital domain.They are mathematically simple and exploit samples during the life time of the pulse, hence appropriate for field measurements.

Several new methods for the digital discrimination of neutrons and gamma-rays in a mixed radiation field are presented. The methods introduced discriminate neutrons and gamma rays successfully in the digital domain. They are mathematically simple and exploit samples during the life time of the pulse, hence appropriate for field measurements. All these methods are applied to a set of mixed neutron and photon signals from a stilbene scintillator and their discrimination qualities are compared.

Fig4: The discrimination at 4 % threshold level, illustrating the problem with “Rise-Time” method. The digitizer used is DP210, with 8-bit resolution, set at 1 GS/s

Mentions:
There are two problems associated with the rise-time method. First, the appropriate threshold level for discrimination varies for different data sets and the range of the suitable threshold levels for high-quality discrimination is very narrow. The best threshold level for a data set can be found through trial and error. Therefore this method is not robust. The second problem is that moving the threshold level up or down, even in small steps, could give ambiguous or non-qualified results. If the threshold level is selected too low, while the neutrons and photons fall in separate areas in the discrimination plot, each area itself could be divided into two other areas. This happens even when the data is smoothed enough. Based on our experiments, this problem exists regardless of the resolution or the sampling frequency of the digitizer. For our sample pulses, 2 % threshold level discriminates without this problem, as shown in Fig. 3, however, moving the level to 4 % gives the output shown in Fig. 4. Only through comparing with the other plots obtained at different threshold levels, do we find out that the pulses to the right of 60 discrimination value in Fig. 4 account for neutrons and the ones to the left of it account for photons. However, it is not possible to notice this discrimination by this plot alone. If we move the threshold level higher, the discrimination quality becomes too low to help us detect the two areas for the pulses. Table 1 compares the effect of some threshold-level selections for this experiment. The best discrimination takes place around 5 % threshold level. The cases where there is no proper discrimination, including the second problem mentioned above, are marked with N/A.Fig. 3

Fig4: The discrimination at 4 % threshold level, illustrating the problem with “Rise-Time” method. The digitizer used is DP210, with 8-bit resolution, set at 1 GS/s

Mentions:
There are two problems associated with the rise-time method. First, the appropriate threshold level for discrimination varies for different data sets and the range of the suitable threshold levels for high-quality discrimination is very narrow. The best threshold level for a data set can be found through trial and error. Therefore this method is not robust. The second problem is that moving the threshold level up or down, even in small steps, could give ambiguous or non-qualified results. If the threshold level is selected too low, while the neutrons and photons fall in separate areas in the discrimination plot, each area itself could be divided into two other areas. This happens even when the data is smoothed enough. Based on our experiments, this problem exists regardless of the resolution or the sampling frequency of the digitizer. For our sample pulses, 2 % threshold level discriminates without this problem, as shown in Fig. 3, however, moving the level to 4 % gives the output shown in Fig. 4. Only through comparing with the other plots obtained at different threshold levels, do we find out that the pulses to the right of 60 discrimination value in Fig. 4 account for neutrons and the ones to the left of it account for photons. However, it is not possible to notice this discrimination by this plot alone. If we move the threshold level higher, the discrimination quality becomes too low to help us detect the two areas for the pulses. Table 1 compares the effect of some threshold-level selections for this experiment. The best discrimination takes place around 5 % threshold level. The cases where there is no proper discrimination, including the second problem mentioned above, are marked with N/A.Fig. 3

Bottom Line:
Several new methods for the digital discrimination of neutrons and gamma-rays in a mixed radiation field are presented.The methods introduced discriminate neutrons and gamma rays successfully in the digital domain.They are mathematically simple and exploit samples during the life time of the pulse, hence appropriate for field measurements.

Several new methods for the digital discrimination of neutrons and gamma-rays in a mixed radiation field are presented. The methods introduced discriminate neutrons and gamma rays successfully in the digital domain. They are mathematically simple and exploit samples during the life time of the pulse, hence appropriate for field measurements. All these methods are applied to a set of mixed neutron and photon signals from a stilbene scintillator and their discrimination qualities are compared.